Polarization statistics of thermal microwave radiation.

Time series of thermal emissions from water vapor and air molecules at ≈2.8GHz, collected by the National Center for Atmospheric Research's (NCAR) S-band polarimetric radar S-Pol are examined for polarization probability distributions. A nearly uniform distribution of states on the Poincaré sphere surface is found. This uniformity is consistent with the four-dimensional circular Gaussian distribution of electric fields (pairs of in-phase and quadrature components for two orthogonal directions) for unpolarized radiation. Analysis of experimental data and the derived sampling distribution of the degree of polarization of the finite time series both yield a mean of about 0.02, within bounds of sampling variability of unpolarized Gaussian-distributed radiation. Weak inhomogeneity of polarization states on the Poincaré sphere detected in the experimental data is within the error bounds of the radar receiver.